I am getting an odd effect. I am doing some LRGB imaging at the moment and I am getting elongated stars, but oddly, mainly in Green and Blue frames. L & R are unaffected. Couple of thoughts I have had:

1) I am using 31mm unmounted filters. They are held in place with small screws and a rubber washer under each screw. Maybe they are not seated well enough so creating a slight angle.

2) When I was installing them, the instruction required me to work out which side had anti reflective coating which should face the camera. Maybe I installed 2 filters the wrong way around. Not sure how that could cause the issue, but if camera tilt, collimation or guiding is the issue it should be consistent in all images.

Malcolm, I battled with elongated stars for months, I tried everything to fix it and in the end it came down to a lens in a coma corrector that had a very slight tilt in it....wasnít seated properly. The tilt was only a fraction of a mm however it was enough to split the stars in my image into a slight spectrum, which ended up looking like elongated stars unless you looked really closely.

My advice - check everything is seated correctly and your lens/filters arenít playing games with the light path

- Have you tried them one after another? Iím wondering if it could be something mechanical that changes over time Iíd you did LRGB in that order over the course of the night.

- Do you refocus between filter changes or use filter offsets? Sometimes slight refocus can cause non round stars.

Thanks Col
Had 2 consecutive images (end of run with one filter and start of run with next) that were completely different. Focus adjustment was tiny, only a few steps and this focuser uses steps of 0.2667 microns. Of course, it does rack in and out a bit while refocussing (I have it set to refocus on each filter change as have not yet managed to get filter offsets to work, must try again on that one!)
One thing I did find while packing up last night. The focuser (it's a Moonlite Litecrawler) has an adapter to take the 68mm thread in the focuser down to 42 and it has a locking ring on it. That was wound out a little bit so may be causing a bit of sag. Cannot see how that would very between filters though.